Carbon fibers have been used in various composite materials because of their properties such as excellent mechanical properties and high electrical conductivity.
On the other hand, in recent years, higher functionalities have come to be required for various materials. Additives which can greatly improve properties, such as electrical, mechanical and thermal properties, of a matrix comprising solid materials such as resins, ceramics and metals, without damaging the characteristics of the matrix have been sought after. Additionally, additives which can improve physical properties of liquids, such as fuel oil and lubricants, have also been sought after.
As the carbon fibers, fine carbon fibers such as carbon nano structures exemplified by the carbon nanotubes (hereinafter, referred to also as “CNT”), have been attracting public attention in various fields.
Graphite layers that make up the carbon nano structures are materials normally comprised of regular arrays of six-membered rings whose structures can bring about unique electrical properties, as well as chemically, mechanically, and thermally stable properties. Therefore, if the foregoing properties can be made use of by incorporating and dispersing such fine carbon fibers into solid materials, including various resins, ceramics, metals, etc., or into liquid materials, including fuels, lubricant agents, etc., their application as additives can be expected.
However, on the other hand, such fine carbon fibers tend to aggregate even just after their synthesis. If these aggregates are used as-is, it may result in insufficient dispersion in a matrix, thereby leading to poor performance. Accordingly, in order to impart a desired property such as electrical conductivity to a matrix such as a resin, it is necessary that the fine carbon fibers would be added in a large amount.
Patent Document 1 discloses a fine carbon fibrous structure which comprise a three dimensional network of fine carbon fibers, each of which having an outside diameter of from 15 to 100 nm. The fine carbon fibrous structure comprises a granular part, at which the plural fine carbon fibers are tied together in a state such that a plurality of the fine carbon fibers extend outwardly from the granular part. The particle diameter of the granular part is larger than the outside diameter of the fine carbon fiber, and the granular part is formed in the following growth process: by using at least two carbon compounds having different decomposition temperatures as carbon sources, growing carbon material in the circumferential direction of a catalyst particle used as well as allowing it to undergo a fibrous growth. Further, the fine carbon fibrous structure has a resistance of 0.02Ω·cm or less measured with powder at the compression density of 0.8 g/cm3. However, as is clear from the description of the Example, etc. of Patent Document 1, the outside diameter of fine carbon fibrous structure is 100 nm or less.
Patent Document 2 discloses a fine carbon fibrous structure composed of carbon fibers having an outside diameter of 100 nm or less, wherein the fine carbon fibers and granular parts are continuous, and the length of carbon fibers between two granular parts is 20 μM or less.
Patent Document 3 discloses a resin composition containing aggregates, wherein each of the aggregate is composed of mutually entangled carbon fibrils having a diameter from 3.5 to 70 nm, and wherein the aggregates possess a diameter in the range of 0.10 to 0.25 mm with a maximum diameter of not more than 0.25 mm. However, as is clear from the descriptions in the Examples, etc. of Patent Document 3, the disclosed values of the maximum diameter and diameter of the carbon fibril aggregate are characteristic values of the aggregates before incorporation into a resin.
Further, Patent Document 4 discloses a composite material comprising a carbon fibrous material incorporated into the matrix. The carbon fibrous material mainly comprises a structure comprising aggregates of carbon fibers having a diameter of 50 to 5000 nm. The mutual contacting points among the carbon fibers are fixed with carbonized carbonaceous substance. The diameter of the structure has a size of 5 μm to 500 μm. Also in Patent Document 4, the disclosed values of the structure size, etc. are characteristic values before incorporation into a resin.
Thus, developments of carbon fibers with various shapes have been made, and carbon fibers suitable for each application have been demanded. However, the carbon fibrous structures described in Patent Document 1 and Patent Document 2 have an outside diameter of 100 nm or less, and there is no disclosure therein regarding a production method without causing variations in electrical conductivity etc. obtained after addition into a resin.
Moreover, the aggregates described in Patent Document 3 are obtained by dispersing carbon fibrils under a shearing force, such as in a vibrating ball mill or the like. However, they have high bulk densities. Thus, they are not satisfactory yet as an additive for improving properties such as electrical conductivity effectively by a small amount addition.
Regarding the carbon fibrous structure described in Patent Document 4, fixing at the contacting points among carbon fibers is formed by conducting heat treatment after production of carbon fibers in a state such that mutual contacting points among the carbon fibers are formed by compression-molding the produced carbon fiber aggregates, and carbonizing organic residues such as pitch remaining on the surface of the carbon fibers, or carbonizing an organic compound added as a binder. Therefore, the fixing forces at the contacting points are weak, and the electrical properties of the structure per se cannot be considered as so good. Accordingly, in the case that these carbon structures are added to a matrix such as a resin, the contacting points are easily detached and the shape of the structure cannot be kept. For this reason, for example, it is difficult to form satisfactory conductive paths in a matrix for exerting good electrical properties, by a small amount addition thereof. Furthermore, when a binder or the like is added to fix the contacting points, followed by carbonization, it is difficult to allow the binder to attach only to the sites of the contacting points. Because the binder attaches to the whole surface area of the fibers, it is highly likely that this technique might provide only structures having large fiber diameter as a whole and poor surface characteristics.